From: Robert Klemme <shortcutter@...>
Date: 2010-10-31T21:40:13+09:00
Subject: Re: Dynamically reference instance vars

On 30.10.2010 22:11, Greg Willits wrote:
> Ryan Davis wrote in post #958245:
>> On Oct 30, 2010, at 12:27 , Greg Willits wrote:
>>
>>> I was hoping in Ruby that at least this was possible:
>>>
>>>   myobject.send(iname) = ivalue
>>>   --or--
>>>   myobject.send(iname, ivalue)
>>
>> You're confusing methods and variables. They're not the same, but you're
>> on the right track with the code above, but you'd be calling the getter,
>> not the setter. Check it:
>>
>> class X
>>    attr_accessor :x # creates x and x= methods
>> end
>>
>> o = X.new
>> o.send("x=", 42)
>> p x
>
> I wasn't confusing them, I was hoping Ruby's send would be flexible
> enough to work with both of them (in conjunction with the accessors
> being defined).  What I didn't think of in this context was Ruby's x=
> being considered the setter method and not just x, which I knew, and
> should have recognized -- so yep, that was the ticket. You win! :-)
> Sorry, no prizes :-(

Here are two other approaches using widely underused class Struct and 
that differ in the way they deal with values not present in the Hash:

Shape = Struct.new :size, :fill_color, :line_color, :line_width do
   def self.from_hash_1(h)
     sh = new
     h.each {|k,v| sh[k] = v}
     sh
   end

   def self.from_hash_2(h)
     sh = new
     members.each {|m| sh[m] = h[m]}
     sh
   end
end

irb(main):015:0* s1 = Shape.from_hash_1(
irb(main):016:1*   :size => 'small',
irb(main):017:1*   :fill_color => 'red',
irb(main):018:1*   :line_color => 'black',
irb(main):019:1*   :line_width => '2'
irb(main):020:1> )
=> #<struct Shape size="small", fill_color="red", line_color="black", 
line_width="2">
irb(main):021:0> s2 = Shape.from_hash_2(
irb(main):022:1*   :size => 'small',
irb(main):023:1*   :fill_color => 'red',
irb(main):024:1*   :line_color => 'black',
irb(main):025:1*   :line_width => '2'
irb(main):026:1> )
=> #<struct Shape size="small", fill_color="red", line_color="black", 
line_width="2">

If you like that better you can as well define

def Shape(h)
   sh = Shape.new
   # logic from above
   sh
end

Then you can do

s1 = Shape(
   :size => 'small',
   :fill_color => 'red',
   :line_color => 'black',
   :line_width => '2'
)

Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/